A back pressure control type EGR system is provided with a vacuum-operated flow control valve located on a recirculation passageway connecting an exhaust manifold of the engine with an intake manifold of the engine, and a vacuum modulator valve having a control chamber normally opened to the atmosphere. The control chamber is, in response to the pressure of the exhaust gas in a small space formed in the recirculation passageway, opened to a vacuum line connecting the flow control chamber with an EGR port formed in an intake passageway of the engine. This system makes it possible to maintain a predetermined constant pressure of the exhaust gas in the constant pressure space, which pressure is near atmospheric air pressure, as is well known to those skilled in this art. Thus, a predetermined constant ratio of the amount of the recirculated exhaust gas to the total amount of fluid directed to the engine combustion chamber (so-called EGR ratio) is obtained at every throttle opening or load of the engine.
The prior art back pressure control type EGR system has, however, a drawback in that an adverse effect, such as "surging", easily takes place under a low load condition of the engine, when a large amount of exhaust gas is recirculated for effectively decreasing the amount of NO.sub.x components in the exhaust gas. This is because a maximum value of the EGR ratio where the surging does not occur is low when the load of the engine is small, while the actual EGR ratio is, irrespective of the load of the engine, maintained at a predetermined value as already described.